Fast Degradation for High Activity: Oxygen- and Nitrogen-Functionalised Carbon Nanotubes in Solid-Acid Fuel-Cell...

Similar to polymer electrolyte membrane fuel cells, the widespread application of solid acid fuel cells (SAFCs) has been hindered partly by the necessity of the use of the precious-metal catalyst Pt in the electrodes. Here we investigate multi-walled carbon nanotubes (MWCNTs) for their potential catalytic activity by using symmetric cell measurements of solid-acid-based electrochemical cells in a cathodic environment. For all measurements, the carbon nanotubes were Pt free and subject to either nitrogen or oxygen plasma treatment. AC impedance spectroscopy of the electrochemical cells, with and without a DC bias, was performed and showed significantly lower initial impedances for oxygen-plasma-treated MWCNTs compared to those treated with a nitrogen plasma. In symmetric cell measurements with a DC bias, the current declines quickly for oxygen-plasma-treated MWCNTs and more slowly, over 12 days, for nitrogen-plasma-treated MWCNTs. To elucidate the degradation mechanisms of the oxygen-plasma-treated MWCNTs under SAFC operating conditions, theoretical calculations were performed using DFT. The results indicate that several degradation mechanisms are likely to occur in parallel through the reduction of the surface oxygen groups that were introduced by the plasma treatment. This finally leads to an inert MWCNT surface and a very low electrode performance. Nitrogen-plasma-treated MWCNTs appear to have a higher stability and may be worthwhile for future investigations.